Microfluidic technology is used in systems that perform chemical and biological analysis, as well as chemical synthesis, on a much smaller scale than previous laboratory equipment and techniques. Microfluidic systems offer the advantages of only requiring a small sample of analyte or reagent for analysis or synthesis, and dispensing a smaller amount of waste materials. A microfluidic system may be a single component or part of a larger system. For example, the system may include interface elements such as an electrospray ionization tip, which would allow the system to be interfaced to a mass spectrometer. The term “microfluidic” as used herein refers to features that are fabricated on the micron or submicron scale. For example, a typical channel or chamber of a microfluidic system has at least one cross-sectional dimension in the range of approximately 0.1 microns to 1000 microns.
As microfluidic systems increase in complexity, the importance of reliable electronic and software processing support to enhance the analysis capabilities also increases. Known microfluidic systems provide processing support for performing operations such as measuring the flow rate of fluid through a system passageway. An accurate determination of flow rate is important in a number of different applications, such as in HPLC (High Performance Liquid Chromatography) coupled to mass-spectrometer analysis.
U.S. Pat. No. 6,386,050 to Yin et al. describes a system and method for measuring flow rate within a fluid-bearing passageway of a microfluidic system. Heat tracers (i.e., thermal fluctuations) are introduced into the flow, so that passage to an interrogation region may be detected and timed. The heat tracers may be introduced using an optical heat generator or an electrical element, such as a heating resistor. Optical or electrical properties of the fluid may be monitored to detect passage of heat tracers into the interrogation region. The Yin et al. patent teaches that the flow rate is based directly upon calculating the speed of a heat tracer.
A number of other systems for measuring flow rates are described in the Yin et al. patent. For example, in the described system of U.S. Pat. No. 4,938,079 to Goldberg, intrusive monitoring is used. In the Goldberg system, the heat tracer is introduced into the flow of liquid by a microwave heating device or a source of focused infrared energy. The flow rate is measured by determining the transit time of the heat tracer from its source to a sensor. As one possibility, the dielectric constant of the liquid is monitored to detect changes in temperature. The Goldberg system is designed to provide accuracy at flow rates of less than 100 cc/hour. U.S. Pat. No. 5,726,357 to Manaka and U.S. Pat. No. 5,623,097 to Horiguchi et al. also describe microfabricated devices which employ thermal approaches to calculating flow rates.
While the known approaches to calculating flow rates based on measuring the time interval required for a heat tracer to travel a particular distance provide useful information, more accurate calculations are sought.